1. Field of the Invention
The present invention relates to a composition based on propylene polymers exhibiting an improved compromise between rigidity and weldability. It relates more particularly to a composition suitable for use in making numerous articles such as films, sheets or expanded granules as well as to a process for preparation of such a composition.
2. Discussion of the Background
It is known that propylene polymers can be used for numerous applications such as the manufacture of preformed articles, films, sheets and cellular articles. In these applications, polypropylene is preferred to other plastic materials by virtue of its chemical resistance, its heat resistance and/or its impact resistance and rigidity.
For example, European Patent Application 0860457 describes the use of a propylene block copolymer containing 60 to 90 wt % of a propylene homopolymer and 40 to 10 wt % of a statistical propylene copolymer containing 1 to 10 wt % of ethylene for the manufacture of molded articles such as, in particular, bioriented bottles. The preferred compositions, which contain more than 75 wt % of homopolymer, do not have sufficient weldability, however, for applications such as films or cellular articles.
In addition, European Patent Application 0893470 discloses the use, for the manufacture of expanded granules, of compositions containing 60 to 90 parts by weight of a propylene homopolymer having a melt flow index (MFI) ranging from 0.1 to 20 g/10 minutes and 40 to 10 parts by weight of a statistical propylene copolymer which contains 3 to 20 mol % of a comonomer such as ethylene and which has an MFI of 25 to 3000 g/10 minutes. However, the use of the preferred compositions, in which the ratio of the MFI of the copolymer to that of the homopolymer is greater than 20, leads to heterogeneities. In addition, the use of such resins is economically disadvantageous because of their high manufacturing cost.
Finally, German Patent Application 4420590 discloses polyolefin foam particles of uniform density obtained from a mixture compatible with at least two different propylene polymers, wherein the majority of the mixture is formed from propylene copolymers and terpolymers, at least one of which contains 1-butene. Again, the high cost of the compositions due to the butene copolymer or copolymers (low polymerization yield, cost of monomer recycling installations) prevents their widespread use.
Accordingly, one object of the present invention is to provide a composition which does not suffer from the disadvantages of the conventional compositions.
Another object of the present invention is to provide a composition having improved processability.
Another object of the present invention is to provide a composition having a good balance of mechanical properties and sealing properties.
These and other objects have now been achieved by the present invention, the first embodiment of which provides a composition, which includes:
(a) from 10 to 55 parts by weight of a propylene polymer (A) which includes up to 2% by weight of ethylene and having a melt flow index MFI(A) of 0.1 to 30 g/10 minutes, measured according to ASTM D 1238 (1986) under a load of 2.16 kg at 230xc2x0 C.; and
(b) from 90 to 45 parts by weight of a polymer (B), which is a statistical copolymer of propylene which includes from 1 to 10% by weight of ethylene and having a melt flow index MFI(B) of 0.1 to 30 g/10 minutes;
wherein the ethylene concentration of polymer (B) is greater than that of polymer (A).
Another embodiment of the invention provides an article, which includes the above-described composition.
Another embodiment of the invention provides a process for preparing the above-described composition, which includes two successive polymerization stages, wherein 10 to 55 parts by weight of polymer (A) and from 90 to 45 parts by weight of polymer (B) are respectively prepared.